Electrical discharge machining



u y 3, 1956 c. MCKECHNIE 2,753,429

ELECTRICAL DISCHARGE MACHINING Filed Feb. 25, 1955 1 i r U INVENTOR.

iTTOF/VZ'E United States Patent ELECTRICAL DISCHARGE MACHINING Ian C.McKechnie, Pontiac, Mich., assiguor to Elox Corporation of Michigan,Clawson, Mich.

Application February 25, 1953, Serial No. 338,670

4 Claims. (Cl. 219-) This invention pertains to a method and apparatusfor removing material from electrically conductive stock byelectrical-discharge-machining More specifically it relates to aself-contained electricaldischarge-machining apparatus which is simplein design, economical in manufacture when compared to apparatus nowavailable, and which can be operated ofi a standard 60 cycle supplywithout use of rectifiers or other auxiliary a paratus.

The object of the invention is to provide an apparatus of the type abovedescribed.

For a description and illustration of a preferred embodiment of theinvention, reference is made to the accompanying drawings in whichreference characters have been used in the several views to designateparts referred t0.

In the drawings,

Fig. 1 is a side elevational view of the dischargemachining apparatuswith certain parts broken away to show details thereof and theelectrical connections shown diagrammatically;

Fig. 2 is an enlarged sectional view of a portion of the Fig. 1apparatus; and

Fig. 3 is an enlarged sectional View of the electrode entering the work.

Referring now to the views and to Fig. 1 in particular, it may be seenthat the apparatus comprises a supporting structure or frame it havingan upward extension 12 on which is mounted an adjustable drill presshead or other suitable support 14. The vibrating head 16 of the tool ismounted on the rack-bar 1S ordinarily provided and is thereforeadjustable vertically by manipulation of a handle 24 An adjustable shelf22 is provided for supporting a workpiece such as the one showndesignated by numeral 24.

The vibratory head 16 of the apparatus is mounted in a casing 26 whichconsists of a pair of cylindrical housings having mating flanges adaptedto be secured together by bolts 28. The casing 26 is secured to therack-bar 18 by suitable fastening means and has an aperture 30 foraccommodating the vibratory rod 32.

The rod 32 carries an armature 34 at its upper end. The latter comprisesa pair of permanent magnets 36 of opposite polarity spaced apredetermined distance apart. A field structure 38 is disposed insurrounding relation to the armature 34 and includes a field coil 40adapted for connection to an A. C. supply by means of leads 42.

The field structure has a bell-shaped lower extension 44 which mateswith a second bell-shaped member 46, the latter having a cylindricalextension 48 which slidably guides the rod 32. Each of the bell-shapedmemhers is provided with a flange 50, 52, which flanges are disposed inmating relationship as shown.

A flexible metallic diaphragm 54 is secured between the flanges 50, 52at its marginal portion and at its center has an opening which receivesthe rod 32. A collar 56 and a nut 58 carried by the rod constitute meansfor secur- "ice ing the rod to the diaphragm such that the rod andarmature are supported for vertical vibratory motion.

At its lower end the rod 32 carries a fluid connection block 60 which isprovided with a fluid passage through which fluid may flow from asuitable supply 62. Below the connection block, the rod carries a chuck64 which is adapted to mount an electrode 66. The electrode may be ofany desired size and has a hollow passageway 68 through which fluid mayflow from the supply 62.

Figs. 1 and 3 show a typical workpiece which consists of a fabricatedpart 24 in which a broken tap or screw 68 has become imbedded.

In the operation of the machine, the coolant supply 62 is turned on, theswitch (not shown) energizing the field coil is closed and a source ofE. M. F. 70 is connected across the work and the electrode by means ofleads 72 and 74. The E. M. F. source '76 may be the same source as usedto energize the field coil or a separate source may be provided. Thecoolant may be tap water, kerosene or a mixture. The coolant serves tocool the electrode and the area of the work adjacent the discharge areaand expels the particles removed from the work by the action of theelectrical discharge.

After the apparatus has been activated as above described, the electrode66 is moved into proximity with the work by manipulation of the handle29. The armature along with the rod 32 and the electrode 66 will vibrateat power line frequency which, in most cases, wil be 60 cycles per seconBy properly adjusting the handle 20 with regard for the length of thevibratory stroke as determined by the constants of the flexiblediaphragm 54, the electrode 66 may be positioned such that it willapproach the work closely enough to permit an electrical dischargebetween the electrode and the work at the bottom of its stroke andrecede sufficiently at the top of its stroke to prevent a dischargetaking place when the electrode is positive relative to the workpiece.

Thus it will be seen that an electric arc or spark discharge will occurbetween the electrode and work once each cycle and the work will beeroded, the eroded particles being flushed away by the coolant flow asindicated diagrammatically in Fig. 3.

For maximum cutting efliciency it is desirable that the work be positiveand the electrode negative when the discharge takes place. In theapparatus being described, A. C. current is used in the interest ofeconomy and simplicity. It is therefore necessary to properly phase themechanical motion of the tool with the power line frequency such thatthe work will be positive and the tool negative when the tool is at thebottom of its stroke, or at a define point with relation to the bottomof the stroke. This can be accomplished in several ways. The resonantfrequency of the vibratory system can be fixed by carefully choosing theconstants of the diaphragm 54. Under some conditions, it may bedesirable to employ a phase-- shifting network having the requiredamount of inductancc, resistance or capacity to effect the desiredresult. My issued Patent No. 2,501,954, dated March 28, 1950,illustrates and describes the use of springs and networks for thispurpose and reference is made thereto. Of course it is necessary toconnect the leads to the power supply 7 or suitable alternate phasedsupply in a polarized manner so as to result in the desired displacementof the electrode relative to the power pulse within electrical degrees.

Instead of a pair of spaced permanent magnets 36, a single magnet may beused with its north and south poles properly oriented. Similarly, i mayuse a pair of diaphragms instead of the single diaphragm shown.

While I have described but one of the many forms the invention may take,it will be understood that this 3 has been done for illustrativepurposes and that it is not desired or intended to limit the inventionin its details or application except as set forth in the appendedclaims.

Lhave described the use of my method and apparatus as a tap buster orsalvage machine. Obviously, however, the method and apparatus are usefulin many other applications such as machining hard alloys, carbides, etc.

I claim:

1. In an apparatus for machining a conductive workpiece, an electrode, ahead for reciprocating said electrode comprising an annular fixed coil,an armature having spaced permanent magnets disposed in the flux path ofsaid coil, a yieldable member mounting said armature, an extensionmember connecting said electrode to said armature, means connecting saidelectrode and the workpiece to an A. C. source, and means connectingsaid coil to'said A. C. source.

2. In an apparatus for machining a conductive work piece by intermittentelectrical discharges, a source of A. C. power, an electrode; meansconnecting said electrode and the workpiece to said A. C. source, meansfor reciprocating said electrode at A. C. power supply frequencycomprising a fixed coil of ring form, means connecting said coil to saidA. C. source, a rod adapted to mount said electrode, an armature carriedon said rod and disposed Within said coil, said armature comprising apair of permanent magnets of opposite polarity; and a yieldable membermounting said rod for axial reciprocation.

3. An apparatus for machining a conductive workpiece comprising, anelectrode, means connecting said electrode to one side of an A. C.source, means for connecting a workpiece to the other side of saidsource, a fixed coil of annular shape, means connecting said coil tosaid source,

. an armature comprising a pair of permanent magnets of References Citedin the file of this patent UNITED STATES PATENTS 458,872 Depoele Sept.1, 1891 2,167,078 Lakatos July 25, 1939 2,399,977 Bardos May 7, 19462,501,954 McKechni'e Mar. 28, 1950 -lul

